With the board I use it has a setting in the Security/Policy section for setting new user validations. I have it set to Admin which means anyone can apply for membership and they get a validating status that shows them as a new member but does not allow them to post until I manually validate them.

New registration email validation?
Make admin manually preview all new accounts

No validation can work also if you set up a new group that only allows them to post in one forum. You can create a forum called new members. In this forum you can create one thread. You can then give the new member posting rights but only to this forum and disallow starting new threads. Then make the forum only visible to you and new members. This way if it's a valid member you can change the members group to the usual group.